1. Field of the Invention
The present invention relates to a glass window antenna system for motor vehicles, particularly to increase the sensitivity for an AM band in the system which may receive both AM and FM bands.
2. Description of the Prior Art
As a conventional glass window antenna system for motor vehicles which may receive both AM and FM bands, there is a glass window antenna system in which antenna patterns are provided in proximity to a defogging device (hereinafter referred to as "defogger") so as to be capacitively coupled thereto. The defogger is consisted of heater wires and bus-bars provided on a rear window, the bus-bars applying a current to the heater wires. A choke coil is provided between the bus-bars and a DC power supply for the defogger.
This type of conventional glass window antenna system has various problems such as the decrease of S/N ratio due to an engine noise for the defogger, the decrease of a sensitivity for a FM band due to an interference between the antenna patterns and the defogger, and the difficulty for making the antenna system compact due to the big and heavy choke coil.
In order to dissolve these problems, there is provided a glass window antenna system for motor vehicles which may receive both AM and FM bands in Japanese laid-open publication No. 9-181513. There is shown in FIG. 1 the antenna system disclosed in this publication. The antenna system comprises a glass plate of a rear window 1, a circuit mounting component 2, an antenna terminal/power supply terminal box 3, a junction box for a second AM antenna 4, a bus-bar 5, a FM antenna 6, a first AM antenna 7, a second AM antenna 8, a receiver 9, a heater wire 10, a flexible circuit board 11, and a DC power supply 12.
FIG. 2 shows a circuitry of the circuit mounting component 2. The circuitry comprises an AM resonance inductor 13, a damping resistor 14, a high frequency inductor 15 to compensate the decrease of an impedance in an AM band wherein the AM resonance inductor 13 becomes capacitive, a damping resistor 16, an AM resonance inductor 17, a coupling capacitor 18, a connection line 19 on the flexible circuit board 11.
According to this glass window antenna system for motor vehicles, the first and second AM antennas 7, 8 provided on the glass plate 1 are connected together by the line 19 on the flexible circuit board 11. As a result, a series resonance is caused by the stray capacitance for the AM antennas 7, 8 and the inductance of the AM resonance inductor 13, and a parallel resonance is caused by the stray capacitance for AM antennas 7, 8 and the inductance of the AM resonance inductor 17. By these two kinds of resonance, i.e. the series resonance and parallel resonances, a flat sensitivity characteristic is achieved for one received frequency band. Therefore, both the AM antennas 7, 8 and the FM antenna 6 may be used for receiving an AM broadcast while increasing the sensitivity thereto.
The conventional glass window antenna system disclosed in the Japanese laid-open publication No. 9-181513 has utilized both series and parallel resonances for receiving an AM band, so that it is difficult to set appropriately the inductance values of the resonance inductors 15, 17 in order to establish both series and parallel resonance conditions, respectively.